Diesel engines, particularly four stroke trunk piston diesel engines, are generally employed as internal combustion engines for generation of electric power and operation of marine vessels. These internal combustion engines are operated smoothly using a lubricating oil composition which contains various additives.
Generally, most additives employed in lubricating oil compositions are metal-containing detergents. Specifically, an over-based metal-containing detergent which neutralizes sulfur oxide produced by the combustion of fuel and disperses combustion deposits such as sludge is generally employed. Particularly, a lubricating oil composition for marine diesel engines which is operated using high sulfur content fuel such as A-heavy oil or C-heavy oil should contain over-based metal-containing detergents. Most metal-containing detergents are alkaline earth metal sulfonates, alkaline earth metal phenates, and alkaline earth metal salicylates. An ashless dispersant such as succinimide is also generally employed. Examples of other generally employable additives include a zinc dialkyldithio-phosphate compound which serves as an anti-oxidation agent and an extreme-pressure agent and an organic anti-oxidation agent such as phenol compound and amine compound.
Recently, the operation conditions of diesel engines for generation of electric power and operation of marine vessel have been getting severe. In addition, these diesel engines are forced to employ a low grade fuel having increased sulfur content. Moreover, the recent demand for economical operation and increase of maintenance efficiency requires extended service periods for using a lubricating oil with no substantial exchange. Therefore, the load applied to the lubricating oil has increased recently.
The lubricating oil is consumed during the operation of engines. Although a fresh lubricating oil is supplemented periodically to compensate the consumed portion, the amount of sulfur oxide compounds and combustion residues produced and deposited in the remaining lubricating oil increases. Therefore, after the engine is operated continuously for a long period of time, the lubricating oil suffers from deterioration and viscosity increase.
Japanese Provisional Patent Publication 2000-63867, U.S. Pat. No. 6,262,001 B1 and Japanese Provisional Patent Publication 2000-87066 disclose a metal-containing overbased calcium carboxylate (i.e., overbased calcium hydrocarbyl-salicylate carboxylate) having high thermal stability and high detergency. It can be prepared with low production costs. These publications describe a variety of compositions for engine lubricating oils.